System and method for retrieving certificates associated with senders of digitally signed messages

ABSTRACT

A system and method for retrieving certificates and/or verifying the revocation status of certificates. In one embodiment, when a user opens a digitally signed message, a certificate that is required to verify the digital signature on the message may be automatically retrieved if it is not stored on the user&#39;s computing device (e.g. a mobile device), eliminating the need for users to initiate the task manually. Verification of the digital signature may also be automatically performed by the application after the certificate is retrieved. Verification of the revocation status of a certificate may also be automatically performed if it is determined that the time that has elapsed since the status was last updated exceeds a pre-specified limit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, priorU.S. patent application Ser. No. 10/975,987, filed on Oct. 29, 2004, theentirety of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates generally to the processing of messages, such ase-mail messages, and more specifically to a system and method forretrieving certificate data associated with encoded messages.

BACKGROUND OF THE INVENTION

Electronic mail (“e-mail”) messages may be encoded using one of a numberof known protocols. Some of these protocols, such as Secure MultipleInternet Mail Extensions (“S/MIME”) for example, rely on public andprivate encryption keys to provide confidentiality and integrity, and ona Public Key Infrastructure (PKI) to communicate information thatprovides authentication and authorization. Data encrypted using aprivate key of a private key/public key pair can only be decrypted usingthe corresponding public key of the pair, and vice-versa. Theauthenticity of public keys used in the encoding of messages may bevalidated using certificates. In particular, if a user of a computingdevice wishes to encrypt a message before the message is sent to aparticular individual, the user will require a certificate for thatindividual. That certificate will typically comprise the public key ofthe individual, as well as other identification-related information.Similarly, if a user of a computing device receives a message that hasbeen digitally signed by a particular individual, the user will requirethe proper certificate (comprising a public key) for that individual ifthe user wishes to verify the digital signature on the message. In somevariant systems, a Pretty Good Privacy (PGP) key or some other objectthat serves to bind the sender's identity and a public key would berequired to verify the digital signature of a message.

Typically, in known e-mail applications, if the certificate that isrequired to verify the digital signature of a signed message received bya user is not stored at the user's computing device, the user mayattempt to search for and retrieve the requisite certificate (e.g. froma remote certificate server) by manually opening a different applicationto initiate the search and retrieval of the certificate. The user maythen initiate a verification of the digital signature with the retrievedcertificate through the e-mail application.

Furthermore, even if the certificate required to verify the digitalsignature of a signed message received by a user is already stored onthe user's computing device, the user may wish to determine anup-to-date revocation status for that certificate. Typically, in knowne-mail applications, the user may initiate a verification of therevocation status of certificates in order to retrieve such informationmanually, by identifying a specific certificate and selecting acorresponding menu option, for example.

SUMMARY OF THE INVENTION

Embodiments of the invention are generally directed to a system andmethod for retrieving certificates and/or verifying the revocationstatus of certificates that automate at least some of the taskstypically performed manually by users in known techniques. Morespecifically, at least some embodiments of the invention may be employedto facilitate the automatic retrieval of at least one of certificatesand the revocation status of certificates associated with senders ofdigitally signed messages, where the receipt of such messages by a usertriggers the automatic retrieval.

In one broad aspect, there is provided a method of retrievingcertificates associated with senders of digitally signed messagesreceived at a computing device, wherein the method is performed by anapplication executing on the computing device, the method comprising thesteps of: detecting when a message comprising a digital signature of thesender of the message is received by a user; identifying a certificateassociated with the sender that comprises a public key capable ofverifying the digital signature; determining whether the certificate isstored on the computing device; and retrieving the certificate from acertificate store remotely located from the computing device if thecertificate is determined to be not stored on the computing device;wherein performance by the application of at least the retrieving stepis triggered by the receipt of the message by the user as detected atthe detecting step.

In another broad aspect, a further step of verifying the digitalsignature may be performed by the application after the certificate isretrieved, without user intervention.

In another broad aspect, a further step of verifying at least onecertificate property of the certificate may be performed by theapplication after the certificate is retrieved, without userintervention. Examples of certificate properties may include the truststatus of the certificate, the expiration status of the certificate, thestrength of the public key of the certificate, and the revocation statusof the certificate.

In another broad aspect, there is provided a method of verifying therevocation status of certificates associated with senders of digitallysigned messages received at a computing device, wherein the method isperformed by an application executing on the computing device, themethod comprising the steps of: detecting when a message comprising adigital signature of the sender of the message is received by a user;identifying a certificate associated with the sender that comprises apublic key capable of verifying the digital signature; and verifying therevocation status of the certificate, wherein the verifying stepcomprises determining an amount of time that has elapsed since therevocation status of the certificate was last verified and retrieving anupdated revocation status for the certificate if the determined amountof time exceeds a pre-specified limit; wherein performance by theapplication of at least the step of verifying the revocation status ofthe certificate is triggered by the receipt of the message by the useras detected at the detecting step.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the invention, and to showmore clearly how it may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings in which:

FIG. 1 is a block diagram of a mobile device in one exampleimplementation;

FIG. 2 is a block diagram of a communication subsystem component of themobile device of FIG. 1;

FIG. 3 is a block diagram of a node of a wireless network;

FIG. 4 is a block diagram illustrating components of a host system inone example configuration;

FIG. 5 is a block diagram showing an example of a certificate chain;

FIG. 6 is a block diagram illustrating components of an example of anencoded message;

FIG. 7A is a flowchart illustrating steps in a method of retrievingcertificates associated with senders of digitally signed messagesreceived at a computing device in an embodiment of the invention;

FIG. 7B is a flowchart illustrating steps in a method of verifying therevocation status of certificates associated with senders of digitallysigned messages received at a computing device in an embodiment of theinvention; and

FIG. 7C is a flowchart illustrating steps in a method of retrievingcertificates associated with senders of digitally signed messagesreceived at a computing device in a variant embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Some embodiments of the invention make use of a mobile station. A mobilestation is a two-way communication device with advanced datacommunication capabilities having the capability to communicate withother computer systems, and is also referred to herein generally as amobile device. A mobile device may also include the capability for voicecommunications. Depending on the functionality provided by a mobiledevice, it may be referred to as a data messaging device, a two-waypager, a cellular telephone with data messaging capabilities, a wirelessInternet appliance, or a data communication device (with or withouttelephony capabilities). A mobile device communicates with other devicesthrough a network of transceiver stations.

To aid the reader in understanding the structure of a mobile device andhow it communicates with other devices, reference is made to FIGS. 1through 3.

Referring first to FIG. 1, a block diagram of a mobile device in oneexample implementation is shown generally as 100. Mobile device 100comprises a number of components, the controlling component beingmicroprocessor 102. Microprocessor 102 controls the overall operation ofmobile device 100. Communication functions, including data and voicecommunications, are performed through communication subsystem 104.Communication subsystem 104 receives messages from and sends messages toa wireless network 200. In this example implementation of mobile device100, communication subsystem 104 is configured in accordance with theGlobal System for Mobile Communication (GSM) and General Packet RadioServices (GPRS) standards. The GSM/GPRS wireless network is usedworldwide and it is expected that these standards will be supersededeventually by Enhanced Data GSM Environment (EDGE) and Universal MobileTelecommunications Service (UMTS). New standards are still beingdefined, but it is believed that they will have similarities to thenetwork behaviour described herein, and it will also be understood bypersons skilled in the art that the invention is intended to use anyother suitable standards that are developed in the future. The wirelesslink connecting communication subsystem 104 with network 200 representsone or more different Radio Frequency (RF) channels, operating accordingto defined protocols specified for GSM/GPRS communications. With newernetwork protocols, these channels are capable of supporting both circuitswitched voice communications and packet switched data communications.

Although the wireless network associated with mobile device 100 is aGSM/GPRS wireless network in one example implementation of mobile device100, other wireless networks may also be associated with mobile device100 in variant implementations. Different types of wireless networksthat may be employed include, for example, data-centric wirelessnetworks, voice-centric wireless networks, and dual-mode networks thatcan support both voice and data communications over the same physicalbase stations. Combined dual-mode networks include, but are not limitedto, Code Division Multiple Access (CDMA) or CDMA2000 networks, GSM/GPRSnetworks (as mentioned above), and future third-generation (3G) networkslike EDGE and UMTS. Some older examples of data-centric networks includethe Mobitex™ Radio Network and the DataTAC™ Radio Network. Examples ofolder voice-centric data networks include Personal Communication Systems(PCS) networks like GSM and Time Division Multiple Access (TDMA)systems.

Microprocessor 102 also interacts with additional subsystems such as aRandom Access Memory (RAM) 106, flash memory 108, display 110, auxiliaryinput/output (I/O) subsystem 112, serial port 114, keyboard 116, speaker118, microphone 120, short-range communications subsystem 122 and otherdevices 124.

Some of the subsystems of mobile device 100 performcommunication-related functions, whereas other subsystems may provide“resident” or on-device functions. By way of example, display 110 andkeyboard 116 may be used for both communication-related functions, suchas entering a text message for transmission over network 200, anddevice-resident functions such as a calculator or task list. Operatingsystem software used by microprocessor 102 is typically stored in apersistent store such as flash memory 108, which may alternatively be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile store such as RAM 106.

Mobile device 100 may send and receive communication signals overnetwork 200 after required network registration or activation procedureshave been completed. Network access is associated with a subscriber oruser of a mobile device 100. To identify a subscriber, mobile device 100requires a Subscriber Identity Module or “SIM” card 126 to be insertedin a SIM interface 128 in order to communicate with a network. SIM 126is one type of a conventional “smart card” used to identify a subscriberof mobile device 100 and to personalize the mobile device 100, amongother things. Without SIM 126, mobile device 100 is not fullyoperational for communication with network 200. By inserting SIM 126into SIM interface 128, a subscriber can access all subscribed services.Services could include: web browsing and messaging such as e-mail, voicemail, Short Message Service (SMS), and Multimedia Messaging Services(MMS). More advanced services may include: point of sale, field serviceand sales force automation. SIM 126 includes a processor and memory forstoring information. Once SIM 126 is inserted in SIM interface 128, itis coupled to microprocessor 102. In order to identify the subscriber,SIM 126 contains some user parameters such as an International MobileSubscriber Identity (IMSI). An advantage of using SIM 126 is that asubscriber is not necessarily bound by any single physical mobiledevice. SIM 126 may store additional subscriber information for a mobiledevice as well, including datebook (or calendar) information and recentcall information.

Mobile device 100 is a battery-powered device and includes a batteryinterface 132 for receiving one or more rechargeable batteries 130.Battery interface 132 is coupled to a regulator (not shown), whichassists battery 130 in providing power V+ to mobile device 100. Althoughcurrent technology makes use of a battery, future technologies such asmicro fuel cells may provide the power to mobile device 100.

Microprocessor 102, in addition to its operating system functions,enables execution of software applications on mobile device 100. A setof applications that control basic device operations, including data andvoice communication applications, will normally be installed on mobiledevice 100 during its manufacture. Another application that may beloaded onto mobile device 100 would be a personal information manager(PIM). A PIM has functionality to organize and manage data items ofinterest to a subscriber, such as, but not limited to, e-mail, calendarevents, voice mails, appointments, and task items. A PIM application hasthe ability to send and receive data items via wireless network 200. PIMdata items may be seamlessly integrated, synchronized, and updated viawireless network 200 with the mobile device subscriber's correspondingdata items stored and/or associated with a host computer system. Thisfunctionality creates a mirrored host computer on mobile device 100 withrespect to such items. This can be particularly advantageous where thehost computer system is the mobile device subscriber's office computersystem.

Additional applications may also be loaded onto mobile device 100through network 200, auxiliary I/O subsystem 112, serial port 114,short-range communications subsystem 122, or any other suitablesubsystem 124. This flexibility in application installation increasesthe functionality of mobile device 100 and may provide enhancedon-device functions, communication-related functions, or both. Forexample, secure communication applications may enable electroniccommerce functions and other such financial transactions to be performedusing mobile device 100.

Serial port 114 enables a subscriber to set preferences through anexternal device or software application and extends the capabilities ofmobile device 100 by providing for information or software downloads tomobile device 100 other than through a wireless communication network.The alternate download path may, for example, be used to load anencryption key onto mobile device 100 through a direct and thus reliableand trusted connection to provide secure device communication.

Short-range communications subsystem 122 provides for communicationbetween mobile device 100 and different systems or devices, without theuse of network 200. For example, subsystem 122 may include an infrareddevice and associated circuits and components for short-rangecommunication. Examples of short range communication would includestandards developed by the Infrared Data Association (IrDA), Bluetooth,and the 802.11 family of standards developed by IEEE.

In use, a received signal such as a text message, an e-mail message, orweb page download will be processed by communication subsystem 104 andinput to microprocessor 102. Microprocessor 102 will then process thereceived signal for output to display 110 or alternatively to auxiliaryI/O subsystem 112. A subscriber may also compose data items, such ase-mail messages, for example, using keyboard 116 in conjunction withdisplay 110 and possibly auxiliary I/O subsystem 112. Auxiliarysubsystem 112 may include devices such as: a touch screen, mouse, trackball, infrared fingerprint detector, or a roller wheel with dynamicbutton pressing capability. Keyboard 116 is an alphanumeric keyboardand/or telephone-type keypad. A composed item may be transmitted overnetwork 200 through communication subsystem 104.

For voice communications, the overall operation of mobile device 100 issubstantially similar, except that the received signals would be outputto speaker 118, and signals for transmission would be generated bymicrophone 120. Alternative voice or audio I/O subsystems, such as avoice message recording subsystem, may also be implemented on mobiledevice 100. Although voice or audio signal output is accomplishedprimarily through speaker 118, display 110 may also be used to provideadditional information such as the identity of a calling party, durationof a voice call, or other voice call related information.

Referring now to FIG. 2, a block diagram of the communication subsystemcomponent 104 of FIG. 1 is shown. Communication subsystem 104 comprisesa receiver 150, a transmitter 152, one or more embedded or internalantenna elements 154, 156, Local Oscillators (LOs) 158, and a processingmodule such as a Digital Signal Processor (DSP) 160.

The particular design of communication subsystem 104 is dependent uponthe network 200 in which mobile device 100 is intended to operate, thusit should be understood that the design illustrated in FIG. 2 servesonly as one example. Signals received by antenna 154 through network 200are input to receiver 150, which may perform such common receiverfunctions as signal amplification, frequency down conversion, filtering,channel selection, and analog-to-digital (ND) conversion. ND conversionof a received signal allows more complex communication functions such asdemodulation and decoding to be performed in DSP 160. In a similarmanner, signals to be transmitted are processed, including modulationand encoding, by DSP 160. These DSP-processed signals are input totransmitter 152 for digital-to-analog (D/A) conversion, frequency upconversion, filtering, amplification and transmission over network 200via antenna 156. DSP 160 not only processes communication signals, butalso provides for receiver and transmitter control. For example, thegains applied to communication signals in receiver 150 and transmitter152 may be adaptively controlled through automatic gain controlalgorithms implemented in DSP 160.

The wireless link between mobile device 100 and a network 200 maycontain one or more different channels, typically different RF channels,and associated protocols used between mobile device 100 and network 200.A RF channel is a limited resource that must be conserved, typically dueto limits in overall bandwidth and limited battery power of mobiledevice 100.

When mobile device 100 is fully operational, transmitter 152 istypically keyed or turned on only when it is sending to network 200 andis otherwise turned off to conserve resources. Similarly, receiver 150is periodically turned off to conserve power until it is needed toreceive signals or information (if at all) during designated timeperiods.

Referring now to FIG. 3, a block diagram of a node of a wireless networkis shown as 202. In practice, network 200 comprises one or more nodes202. Mobile device 100 communicates with a node 202 within wirelessnetwork 200. In the example implementation of FIG. 3, node 202 isconfigured in accordance with General Packet Radio Service (GPRS) andGlobal Systems for Mobile (GSM) technologies. Node 202 includes a basestation controller (BSC) 204 with an associated tower station 206, aPacket Control Unit (PCU) 208 added for GPRS support in GSM, a MobileSwitching Center (MSC) 210, a Home Location Register (HLR) 212, aVisitor Location Registry (VLR) 214, a Serving GPRS Support Node (SGSN)216, a Gateway GPRS Support Node (GGSN) 218, and a Dynamic HostConfiguration Protocol (DHCP) 220. This list of components is not meantto be an exhaustive list of the components of every node 202 within aGSM/GPRS network, but rather a list of components that are commonly usedin communications through network 200.

In a GSM network, MSC 210 is coupled to BSC 204 and to a landlinenetwork, such as a Public Switched Telephone Network (PSTN) 222 tosatisfy circuit switched requirements. The connection through PCU 208,SGSN 216 and GGSN 218 to the public or private network (Internet) 224(also referred to herein generally as a shared network infrastructure)represents the data path for GPRS capable mobile devices. In a GSMnetwork extended with GPRS capabilities, BSC 204 also contains a PacketControl Unit (PCU) 208 that connects to SGSN 216 to controlsegmentation, radio channel allocation and to satisfy packet switchedrequirements. To track mobile device location and availability for bothcircuit switched and packet switched management, HLR 212 is sharedbetween MSC 210 and SGSN 216. Access to VLR 214 is controlled by MSC210.

Station 206 is a fixed transceiver station. Station 206 and BSC 204together form the fixed transceiver equipment. The fixed transceiverequipment provides wireless network coverage for a particular coveragearea commonly referred to as a “cell”. The fixed transceiver equipmenttransmits communication signals to and receives communication signalsfrom mobile devices within its cell via station 206. The fixedtransceiver equipment normally performs such functions as modulation andpossibly encoding and/or encryption of signals to be transmitted to themobile device in accordance with particular, usually predetermined,communication protocols and parameters, under control of its controller.The fixed transceiver equipment similarly demodulates and possiblydecodes and decrypts, if necessary, any communication signals receivedfrom mobile device 100 within its cell. Communication protocols andparameters may vary between different nodes. For example, one node mayemploy a different modulation scheme and operate at differentfrequencies than other nodes.

For all mobile devices 100 registered with a specific network, permanentconfiguration data such as a user profile is stored in HLR 212. HLR 212also contains location information for each registered mobile device andcan be queried to determine the current location of a mobile device. MSC210 is responsible for a group of location areas and stores the data ofthe mobile devices currently in its area of responsibility in VLR 214.Further VLR 214 also contains information on mobile devices that arevisiting other networks. The information in VLR 214 includes part of thepermanent mobile device data transmitted from HLR 212 to VLR 214 forfaster access. By moving additional information from a remote HLR 212node to VLR 214, the amount of traffic between these nodes can bereduced so that voice and data services can be provided with fasterresponse times and at the same time requiring less use of computingresources.

SGSN 216 and GGSN 218 are elements added for GPRS support; namely packetswitched data support, within GSM. SGSN 216 and MSC 210 have similarresponsibilities within wireless network 200 by keeping track of thelocation of each mobile device 100. SGSN 216 also performs securityfunctions and access control for data traffic on network 200. GGSN 218provides internetworking connections with external packet switchednetworks and connects to one or more SGSN's 216 via an Internet Protocol(IP) backbone network operated within the network 200. During normaloperations, a given mobile device 100 must perform a “GPRS Attach” toacquire an IP address and to access data services. This requirement isnot present in circuit switched voice channels as Integrated ServicesDigital Network (ISDN) addresses are used for routing incoming andoutgoing calls. Currently, all GPRS capable networks use private,dynamically assigned IP addresses, thus requiring a DHCP server 220connected to the GGSN 218. There are many mechanisms for dynamic IPassignment, including using a combination of a Remote AuthenticationDial-In User Service (RADIUS) server and DHCP server. Once the GPRSAttach is complete, a logical connection is established from a mobiledevice 100, through PCU 208, and SGSN 216 to an Access Point Node (APN)within GGSN 218. The APN represents a logical end of an IP tunnel thatcan either access direct Internet compatible services or private networkconnections. The APN also represents a security mechanism for network200, insofar as each mobile device 100 must be assigned to one or moreAPNs and mobile devices 100 cannot exchange data without firstperforming a GPRS Attach to an APN that it has been authorized to use.The APN may be considered to be similar to an Internet domain name suchas “myconnection.wireless.com”.

Once the GPRS Attach is complete, a tunnel is created and all traffic isexchanged within standard IP packets using any protocol that can besupported in IP packets. This includes tunneling methods such as IP overIP as in the case with some IPSecurity (IPsec) connections used withVirtual Private Networks (VPN). These tunnels are also referred to asPacket Data Protocol (PDP) Contexts and there are a limited number ofthese available in the network 200. To maximize use of the PDP Contexts,network 200 will run an idle timer for each PDP Context to determine ifthere is a lack of activity. When a mobile device 100 is not using itsPDP Context, the PDP Context can be deallocated and the IP addressreturned to the IP address pool managed by DHCP server 220.

Referring now to FIG. 4, a block diagram illustrating components of ahost system in one example configuration is shown. Host system 250 willtypically be a corporate office or other local area network (LAN), butmay instead be a home office computer or some other private system, forexample, in variant implementations. In this example shown in FIG. 4,host system 250 is depicted as a LAN of an organization to which a userof mobile device 100 belongs.

LAN 250 comprises a number of network components connected to each otherby LAN connections 260. For instance, a user's desktop computer 262 awith an accompanying cradle 264 for the user's mobile device 100 issituated on LAN 250. Cradle 264 for mobile device 100 may be coupled tocomputer 262 a by a serial or a Universal Serial Bus (USB) connection,for example. Other user computers 262 b are also situated on LAN 250,and each may or may not be equipped with an accompanying cradle 264 fora mobile device. Cradle 264 facilitates the loading of information (e.g.PIM data, private symmetric encryption keys to facilitate securecommunications between mobile device 100 and LAN 250) from user computer262 a to mobile device 100, and may be particularly useful for bulkinformation updates often performed in initializing mobile device 100for use. The information downloaded to mobile device 100 may includecertificates used in the exchange of messages. It will be understood bypersons skilled in the art that user computers 262 a, 262 b willtypically be also connected to other peripheral devices not explicitlyshown in FIG. 4.

Furthermore, only a subset of network components of LAN 250 are shown inFIG. 4 for ease of exposition, and it will be understood by personsskilled in the art that LAN 250 will comprise additional components notexplicitly shown in FIG. 4, for this example configuration. Moregenerally, LAN 250 may represent a smaller part of a larger network [notshown] of the organization, and may comprise different components and/orbe arranged in different topologies than that shown in the example ofFIG. 4.

In this example, mobile device 100 communicates with LAN 250 through anode 202 of wireless network 200 and a shared network infrastructure 224such as a service provider network or the public Internet. Access to LAN250 may be provided through one or more routers [not shown], andcomputing devices of LAN 250 may operate from behind a firewall or proxyserver 266.

In a variant implementation, LAN 250 comprises a wireless VPN router[not shown] to facilitate data exchange between the LAN 250 and mobiledevice 100. The concept of a wireless VPN router is new in the wirelessindustry and implies that a VPN connection can be established directlythrough a specific wireless network to mobile device 100. Thepossibility of using a wireless VPN router has only recently beenavailable and could be used when the new Internet Protocol (IP) Version6 (IPV6) arrives into IP-based wireless networks. This new protocol willprovide enough IP addresses to dedicate an IP address to every mobiledevice, making it possible to push information to a mobile device at anytime. An advantage of using a wireless VPN router is that it could be anoff-the-shelf VPN component, not requiring a separate wireless gatewayand separate wireless infrastructure to be used. A VPN connection wouldpreferably be a Transmission Control Protocol (TCP)/IP or User DatagramProtocol (UDP)/IP connection to deliver the messages directly to mobiledevice 100 in this variant implementation.

Messages intended for a user of mobile device 100 are initially receivedby a message server 268 of LAN 250. Such messages may originate from anyof a number of sources. For instance, a message may have been sent by asender from a computer 262 b within LAN 250, from a different mobiledevice [not shown] connected to wireless network 200 or to a differentwireless network, or from a different computing device or other devicecapable of sending messages, via the shared network infrastructure 224,and possibly through an application service provider (ASP) or Internetservice provider (ISP), for example.

Message server 268 typically acts as the primary interface for theexchange of messages, particularly e-mail messages, within theorganization and over the shared network infrastructure 224. Each userin the organization that has been set up to send and receive messages istypically associated with a user account managed by message server 268.One example of a message server 268 is a Microsoft Exchange™ Server. Insome implementations, LAN 250 may comprise multiple message servers 268.Message server 268 may also be adapted to provide additional functionsbeyond message management, including the management of data associatedwith calendars and task lists, for example.

When messages are received by message server 268, they are typicallystored in a message store [not explicitly shown], from which messagescan be subsequently retrieved and delivered to users. For instance, ane-mail client application operating on a user's computer 262 a mayrequest the e-mail messages associated with that user's account storedon message server 268. These messages would then typically be retrievedfrom message server 268 and stored locally on computer 262 a.

When operating mobile device 100, the user may wish to have e-mailmessages retrieved for delivery to the handheld. An e-mail clientapplication operating on mobile device 100 may also request messagesassociated with the user's account from message server 268. The e-mailclient may be configured (either by the user or by an administrator,possibly in accordance with an organization's information technology(IT) policy) to make this request at the direction of the user, at somepre-defined time interval, or upon the occurrence of some pre-definedevent. In some implementations, mobile device 100 is assigned its owne-mail address, and messages addressed specifically to mobile device 100are automatically redirected to mobile device 100 as they are receivedby message server 268.

To facilitate the wireless communication of messages and message-relateddata between mobile device 100 and components of LAN 250, a number ofwireless communications support components 270 may be provided. In thisexample implementation, wireless communications support components 270comprise a message management server 272, for example. Messagemanagement server 272 is used to specifically provide support for themanagement of messages, such as e-mail messages, that are to be handledby mobile devices. Generally, while messages are still stored on messageserver 268, message management server 272 can be used to control when,if, and how messages should be sent to mobile device 100. Messagemanagement server 272 also facilitates the handling of messages composedon mobile device 100, which are sent to message server 268 forsubsequent delivery.

For example, message management server 272 may: monitor the user's“mailbox” (e.g. the message store associated with the user's account onmessage server 268) for new e-mail messages; apply user-definablefilters to new messages to determine if and how the messages will berelayed to the user's mobile device 100; compress and encrypt newmessages (e.g. using an encryption technique such as Data EncryptionStandard (DES) or Triple DES) and push them to mobile device 100 via theshared network infrastructure 224 and wireless network 200; and receivemessages composed on mobile device 100 (e.g. encrypted using TripleDES), decrypt and decompress the composed messages, re-format thecomposed messages if desired so that they will appear to have originatedfrom the user's computer 262 a, and re-route the composed messages tomessage server 268 for delivery.

Certain properties or restrictions associated with messages that are tobe sent from and/or received by mobile device 100 can be defined (e.g.by an administrator in accordance with IT policy) and enforced bymessage management server 272. These may include whether mobile device100 may receive encrypted and/or signed messages, minimum encryption keysizes, whether outgoing messages must be encrypted and/or signed, andwhether copies of all secure messages sent from mobile device 100 are tobe sent to a pre-defined copy address, for example.

Message management server 272 may also be adapted to provide othercontrol functions, such as only pushing certain message information orpre-defined portions (e.g. “blocks”) of a message stored on messageserver 268 to mobile device 100. For example, when a message isinitially retrieved by mobile device 100 from message server 268,message management server 272 is adapted to push only the first part ofa message to mobile device 100, with the part being of a pre-definedsize (e.g. 2 KB). The user can then request more of the message, to bedelivered in similar-sized blocks by message management server 272 tomobile device 100, possibly up to a maximum pre-defined message size.

Accordingly, message management server 272 facilitates better controlover the type of data and the amount of data that is communicated tomobile device 100, and can help to minimize potential waste of bandwidthor other resources.

It will be understood by persons skilled in the art that messagemanagement server 272 need not be implemented on a separate physicalserver in LAN 250 or other network. For example, some or all of thefunctions associated with message management server 272 may beintegrated with message server 268, or some other server in LAN 250.Furthermore, LAN 250 may comprise multiple message management servers272, particularly in variant implementations where a large number ofmobile devices need to be supported.

Embodiments of the invention relate generally to certificates used inthe processing of encoded messages, such as e-mail messages that areencrypted and/or signed. While Simple Mail Transfer Protocol (SMTP),RFC822 headers, and Multipurpose Internet Mail Extensions (MIME) bodyparts may be used to define the format of a typical e-mail message notrequiring encoding, Secure/MIME (S/MIME), a version of the MIMEprotocol, may be used in the communication of encoded messages (i.e. insecure messaging applications). S/MIME enables end-to-end authenticationand confidentiality, and protects data integrity and privacy from thetime an originator of a message sends a message until it is decoded andread by the message recipient. Other known standards and protocols maybe employed to facilitate secure message communication, such as PrettyGood Privacy™ (PGP), OpenPGP, and others known in the art.

Secure messaging protocols such as S/MIME rely on public and privateencryption keys to provide confidentiality and integrity, and on aPublic Key Infrastructure (PKI) to communicate information that providesauthentication and authorization. Data encrypted using a private key ofa private key/public key pair can only be decrypted using thecorresponding public key of the pair, and vice-versa. Private keyinformation is never made public, whereas public key information isshared.

For example, if a sender wishes to send a message to a recipient inencrypted form, the recipient's public key is used to encrypt a message,which can then be decrypted only using the recipient's private key.Alternatively, in some encoding techniques, a one-time session key isgenerated and used to encrypt the body of a message, typically with asymmetric encryption technique (e.g. Triple DES). The session key isthen encrypted using the recipient's public key (e.g. with a public keyencryption algorithm such as RSA), which can then be decrypted onlyusing the recipient's private key. The decrypted session key can then beused to decrypt the message body. The message header may be used tospecify the particular encryption scheme that must be used to decryptthe message. Other encryption techniques based on public keycryptography may be used in variant implementations. However, in each ofthese cases, only the recipient's private key may be used to facilitatedecryption of the message, and in this way, the confidentiality ofmessages can be maintained.

As a further example, a sender may sign a message using a digitalsignature. A digital signature is a digest of the message (e.g. a hashof the message) encoded using the sender's private key, which can thenbe appended to the outgoing message. To verify the digital signature ofthe message when received, the recipient uses the same technique as thesender (e.g. using the same standard hash algorithm) to obtain a digestof the received message. The recipient also uses the sender's public keyto decode the digital signature, in order to obtain what should be amatching digest for the received message. If the digests of the receivedmessage do not match, this suggests that either the message content waschanged during transport and/or the message did not originate from thesender whose public key was used for verification. Digital signaturealgorithms are designed in such a way that only someone with knowledgeof the sender's private key should be able to encode a signature thatthe recipient will decode correctly using the sender's public key.Therefore, by verifying a digital signature in this way, authenticationof the sender and message integrity can be maintained.

An encoded message may be encrypted, signed, or both encrypted andsigned. The authenticity of public keys used in these operations isvalidated using certificates. A certificate is a digital document issuedby a certificate authority (CA). Certificates are used to authenticatethe association between users and their public keys, and essentially,provides a level of trust in the authenticity of the users' public keys.Certificates contain information about the certificate holder, withcertificate contents typically formatted in accordance with an acceptedstandard (e.g. X.509).

Consider FIG. 5, in which an example certificate chain 300 is shown.Certificate 310 issued to “John Smith” is an example of a certificateissued to an individual, which may be referred to as an end entitycertificate. End entity certificate 310 typically identifies thecertificate holder 312 (i.e. John Smith in this example) and the issuerof the certificate 314, and includes a digital signature of the issuer316 and the certificate holder's public key 318. Certificate 310 willalso typically include other information and attributes that identifythe certificate holder (e.g. e-mail address, organization name,organizational unit name, location, etc.). When the individual composesa message to be sent to a recipient, it is customary to include thatindividual's certificate 300 with the message.

For a public key to be trusted, its issuing organization must betrusted. The relationship between a trusted CA and a user's public keycan be represented by a series of related certificates, also referred toas a certificate chain. The certificate chain can be followed todetermine the validity of a certificate.

For instance, in the example certificate chain 300 shown in FIG. 5, therecipient of a message purported to be sent by John Smith may wish toverify the trust status of certificate 310 attached to the receivedmessage. To verify the trust status of certificate 310 on a recipient'scomputing device (e.g. computer 262 a of FIG. 4) for example, thecertificate 320 of issuer ABC is obtained, and used to verify thatcertificate 310 was indeed signed by issuer ABC. Certificate 320 mayalready be stored in a certificate store on the computing device, or itmay need to be retrieved from a certificate store or source (e.g. LDAPserver 284 of FIG. 4 or some other public or private LDAP server). Ifcertificate 320 is already stored in the recipient's computing deviceand the certificate has been designated as trusted by the recipient,then certificate 310 is considered to be trusted since it chains to astored, trusted certificate.

However, in the example shown in FIG. 5, certificate 330 is alsorequired to verify the trust status of certificate 310. Certificate 330is self-signed, and is referred to as a “root certificate”. Accordingly,certificate 320 may be referred to as an “intermediate certificate” incertificate chain 300; any given certificate chain to a rootcertificate, assuming a chain to the root certificate can be determinedfor a particular end entity certificate, may contain zero, one, ormultiple intermediate certificates. If certificate 330 is a rootcertificate issued by a trusted source (from a large certificateauthority such as Verisign or Entrust, for example), then certificate310 may be considered to be trusted since it chains to a trustedcertificate. The implication is that both the sender and the recipientof the message trust the source of the root certificate 330. If acertificate cannot be chained to a trusted certificate, the certificatemay be considered to be “not trusted”.

Certificate servers store information about certificates and listsidentifying certificates that have been revoked. These certificateservers can be accessed to obtain certificates and to verify certificateauthenticity and revocation status. For example, a Lightweight DirectoryAccess Protocol (LDAP) server may be used to obtain certificates, and anOnline Certificate Status Protocol (OCSP) server may be used to verifycertificate revocation status.

Standard e-mail security protocols typically facilitate secure messagetransmission between non-mobile computing devices (e.g. computers 262 a,262 b of FIG. 4; remote desktop devices). Referring again to FIG. 4, inorder that signed messages received from senders may be read from mobiledevice 100 and encrypted messages be sent to those senders, mobiledevice 100 is adapted to store certificates and associated public keysof other individuals. Certificates stored on a user's computer 262 awill typically be downloaded from computer 262 a to mobile device 100through cradle 264, for example.

Certificates stored on computer 262 a and downloaded to mobile device100 are not limited to certificates associated with individuals but mayalso include certificates issued to CAs, for example. Certaincertificates stored in computer 262 a and/or mobile device 100 can alsobe explicitly designated as “trusted” by the user. Accordingly, when acertificate is received by a user on mobile device 100, it can beverified on mobile device 100 by matching the certificate with onestored on mobile device 100 and designated as trusted, or otherwisedetermined to be chained to a trusted certificate.

Mobile device 100 may also be adapted to store the private key of thepublic key/private key pair associated with the user, so that the userof mobile device 100 can sign outgoing messages composed on mobiledevice 100, and decrypt messages sent to the user encrypted with theuser's public key. The private key may be downloaded to mobile device100 from the user's computer 262 a through cradle 264, for example. Theprivate key is preferably exchanged between the computer 262 a andmobile device 100 so that the user may share one identity and one methodfor accessing messages.

User computers 262 a, 262 b can obtain certificates from a number ofsources, for storage on computers 262 a, 262 b and/or mobile devices(e.g. mobile device 100). These certificate sources may be private (e.g.dedicated for use within an organization) or public, may reside locallyor remotely, and may be accessible from within an organization's privatenetwork or through the Internet, for example. In the example shown inFIG. 4, multiple PKI servers 280 associated with the organization resideon LAN 250. PKI servers 280 include a CA server 282 for issuingcertificates, an LDAP server 284 used to search for and downloadcertificates (e.g. for individuals within the organization), and an OCSPserver 286 used to verify the revocation status of certificates.

Certificates may be retrieved from LDAP server 284 by a user computer262 a, for example, to be downloaded to mobile device 100 via cradle264. However, in a variant implementation, LDAP server 284 may beaccessed directly (i.e. “over the air” in this context) by mobile device100, and mobile device 100 may search for and retrieve individualcertificates through a mobile data server 288. Similarly, mobile dataserver 288 may be adapted to allow mobile device 100 to directly queryOCSP server 286 to verify the revocation status of certificates.

In variant implementations, only selected PKI servers 280 may be madeaccessible to mobile devices (e.g. allowing certificates to bedownloaded only from a user's computer 262 a, 262 b, while allowing therevocation status of certificates to be checked from mobile device 100).

In variant implementations, certain PKI servers 280 may be madeaccessible only to mobile devices registered to particular users, asspecified by an IT administrator, possibly in accordance with an ITpolicy, for example.

Other sources of certificates [not shown] may include a Windowscertificate store, another secure certificate store on or outside LAN250, and smart cards, for example.

Referring now to FIG. 6, a block diagram illustrating components of anexample of an encoded message, as may be received by a message server(e.g. message server 268 of FIG. 4), is shown generally as 350. Encodedmessage 350 typically includes one or more of the following: a headerportion 352, an encoded body portion 354, optionally one or more encodedattachments 356, one or more encrypted session keys 358, and signatureand signature-related information 360. For example, header portion 352typically includes addressing information such as “To”, “From”, and “CC”addresses, and may also include message length indicators, and senderencryption and signature scheme identifiers, for example. Actual messagecontent normally includes a message body or data portion 354 andpossibly one or more attachments 356, which may be encrypted by thesender using a session key. If a session key was used, it is typicallyencrypted for each intended recipient using the respective public keyfor each recipient, and included in the message at 358. If the messagewas signed, a signature and signature-related information 360 are alsoincluded. This may include the sender's certificate, for example.

The format for an encoded message as shown in FIG. 6 is provided by wayof example only, and persons skilled in the art will understand thatembodiments of the invention will be applicable to encoded messages ofother formats. Depending on the specific messaging scheme used,components of an encoded message may appear in a different order thanshown in FIG. 6, and an encoded message may include fewer, additional,or different components, which may depend on whether the encoded messageis encrypted, signed or both.

Embodiments of the invention relate generally to the processing ofencoded messages received by a user that have been digitally signed by asender, and are also generally directed to a system and method forretrieving certificates and/or verifying the revocation status ofcertificates that automate at least some of the tasks typicallyperformed manually by users in known techniques.

A digitally signed message that is received by a user of a computingdevice (e.g. mobile device 100) typically contains informationidentifying the signer's certificate (i.e. the certificate of the senderof the message), such as an identification of the certificate's issuerand serial number, or a certificate thumbprint (e.g. a hash ofcertificate data). The identifying information may have been included inthe signed message by the sender of the message, or it may have beenincluded by an intermediate server (e.g. message management server 272)that has performed some pre-processing on the message, for example. Thisinformation allows applications executing on the computing device (e.g.an e-mail application) to identify the correct certificate containingthe public key necessary to verify the digital signature, and todetermine if that certificate is stored on the computing device. Forexample, the certificate may be temporarily stored on the computingdevice if it accompanied the received message, or the certificate may bemore permanently stored on the computing device in a certificate store.

Typically, in known e-mail applications, if the certificate that isrequired to verify the digital signature of a signed message received bya user is not stored at the user's computing device, the user mayattempt to search for and retrieve the requisite certificate (e.g. froma remote certificate server) by manually opening a different applicationto initiate the search and retrieval of the certificate. The user maythen initiate a verification of the digital signature with the retrievedcertificate through the e-mail application.

Even if the certificate required to verify the digital signature of asigned message received by a user is already stored on the user'scomputing device, the user may wish to determine an up-to-daterevocation status for that certificate. Typically, in current e-mailapplications, the user may manually initiate a verification of therevocation status of certificates in order to retrieve such information,by identifying a specific certificate and selecting a corresponding menuoption, for example.

Certain tasks performed in the processing of digitally signed messagesreceived by a user, such as the retrieval of requisite certificates asnoted above, are typically initiated manually by users in known systems.This may be attributed to the fact that when a user wishes to send anencrypted message using public key encryption to another individual, thecertificate (or at least a public key) of that individual is required.In contrast, in situations where a user receives a signed message fromanother individual, the certificate of that individual is not necessaryin order for the user to read the message and understand its contents ifthe message is not encrypted; the certificate is only necessary if theuser wishes to verify the authenticity of the sender or integrity of themessage. Since verification of digital signatures can be performed atthe option of the user, the task of retrieving certificates to performsuch verifications is typically initiated manually.

Moreover, certificates that are issued to individuals are revokedrelatively infrequently. For example, a user's certificate may berevoked if the user leaves an organization, or if the security of theuser's private key has been compromised, for example. These events aregenerally uncommon. As a result, the task of verifying the revocationstatus of certificates is also typically initiated manually by users inknown systems. However, a certificate may be revoked at any time, and ifthe revocation status is not updated sufficiently often, there is a riskthat a digital signature may be verified using an unknowingly revokedcertificate.

The inventors have realized that by automating at least some of thesetasks, additional protection may be afforded to the user, as the usermay inadvertently forget to perform the tasks, and wrongly assume thatthe authenticity of the sender and integrity of a particular message canbe trusted or that a certificate has not yet been revoked. Some userswho find the manual tasks cumbersome or time-consuming may decide not toperform them, choosing instead to risk trusting the authenticity andintegrity of the message. By automating at least some of these tasks,additional protection may be provided while minimizing inconvenience tothe user.

In accordance with at least one embodiment of the invention, when therecipient receives a digitally signed message, an action toautomatically retrieve the certificate required to verify the digitalsignature takes place, if it is determined that the certificate is notavailable locally at the user's computing device. This action istriggered by the receipt of the digitally signed message by the user. Itis performed by an application, such as an e-mail application forexample, without user intervention (i.e. without requiring the user tomanually initiate the search for or retrieval of the certificate). Thisembodiment is described in further detail with reference to FIG. 7Abelow.

For example, this action may include contacting one or more LDAP servers(e.g. LDAP server 284 or some other public or private LDAP server) fromwhich the certificate might be retrieved. The LDAP server(s) to becontacted may be preconfigured on the computing device, for example.

As a further example, if an intermediate server (e.g. message managementserver 272) has performed some pre-processing of the received messageand the requisite certificate is residing on the intermediate server,the action may include contacting the intermediate server to obtain thecertificate. The intermediate server may be optimized to storecertificates retrieved as a result of the pre-processing of messages,but not to forward certificates to the user unless they are specificallyrequested, in order to preserve bandwidth in the event that a givencertificate is already stored on the user's computing device.

In accordance with at least one other embodiment of the invention, whenthe recipient receives a digitally signed message, an action toautomatically verify the revocation status of the message sender'scertificate takes place, if the certificate is already stored on thecomputing device, and if it is determined that the revocation statusrequires updating. This action may be triggered by the receipt of thedigitally signed message by the user. It is performed by an application,such as an e-mail application for example, without user intervention(i.e. without requiring the user to manually initiate the verificationof the revocation status) if the certificate is available at the user'scomputing device (e.g. the certificate was already stored on the user'scomputing device or was successfully retrieved). Accordingly, therevocation status of a sender's certificate may be updated automaticallyevery time a message of the sender is received by the user. Thisembodiment is described in further detail with reference to FIG. 7Bbelow.

In one embodiment, this action may include determining the amount oftime that has elapsed since the revocation status of the sender'scertificate was last verified, and retrieving an updated revocationstatus for the certificate if the determined amount of time exceeds apre-specified limit, from an OCSP server (e.g. OCSP server 286) forexample. A time stamp indicating the time (which typically includes thedate) of a certificate's most recent revocation status verification maybe recorded in a status object stored in a table indexed by certificate(e.g. identified by issuer name and serial number or thumbprint), forexample. This time stamp may be compared to the current time to make therequisite determination.

The pre-specified limit may be established by the user, or by anorganization associated with the user as defined by IT Policy, forexample. A policy file stored on the computing device, for example, maybe employed to facilitate enforcement of the IT Policy on the device.

With respect to at least some of the embodiments described herein, amessage is considered as being received by the user when the user opens(or re-opens) the message for viewing, which is typically done throughan e-mail client application. When the performance of the tasksdescribed above are triggered by the opening of the message for viewingby a user, it is less likely that such tasks would be performedunnecessarily (e.g. if the user ultimately never opens a message thathas arrived at the user's computing device), thereby preservingbandwidth. However, in variant embodiments, a message may be consideredas being received by the user when the message has arrived at the user'scomputing device for storage (e.g. in the user's “inbox”), even thoughthe user may not have actually opened the message yet for viewing.

Referring to FIG. 7A, a flowchart illustrating steps in a method ofretrieving certificates associated with senders of digitally signedmessages received at a computing device in an embodiment of theinvention is shown generally as 400. Further details with respect tovarious steps of method 400 have been discussed in the foregoingdescription.

In method 400, by way of example, the steps are described as beingperformed by an e-mail application that executes and resides on a mobiledevice. However, in variant embodiments, an application other than ane-mail application may perform the steps of the method. Furthermore, invariant embodiments, the application may be residing and executing on acomputing device other than a mobile device operated by a user, or on anintermediate server coupled to a mobile or other computing deviceoperated by a user, for example. The method may also be implementedthrough multiple applications executing and residing on the mobile orother computing device.

At step 410, the e-mail application detects the receipt by a user of adigitally signed message that comprises a digital signature of thesender of the message. In this embodiment, the message is considered asbeing received by the user when the user opens the message.

At step 420, the certificate associated with the sender that comprisesthe public key capable of verifying the digital signature is identifiedby the e-mail application. Information required to identify thecertificate may be provided within the received message. Thisinformation may include an identification of the certificate's issuerand serial number, or the certificate's thumbprint for example.

In this embodiment, this step is performed only after the user opens themessage. However, in a variant embodiment, this step may be performedwhen the message arrives at the device (e.g. when placed in an “inbox”folder for messages) but before the user opens the message. This is sothat when the user actually opens the message, the user need not waitfor this step to complete.

At step 430, the application determines whether the certificateidentified at step 420 is already stored on the mobile device. Forexample, the certificate may be temporarily stored on the mobile deviceif the certificate accompanied the received message, or may be morepermanently stored in a certificate store on the mobile device.

In this embodiment, this step is also performed only after the useropens the message. However, in a variant embodiment, this step may beperformed when the message arrives at the device and after step 420 isperformed, but before the user opens the message. This is so that whenthe user actually opens the message, the user need not wait for thisstep to complete.

At step 440, the application automatically initiates retrieval of therequisite certificate from a certificate store remotely located from thecomputing device if the certificate is not stored on the computingdevice as determined at step 430, upon the user opening the message. Thecertificate store from which an attempt to retrieve the certificate ismade may be one of one or more LDAP servers, or an intermediate server,for example.

Although other intervening steps (e.g. steps 420 and 430) may beperformed between step 410 and step 440, step 440 is effectivelytriggered by the opening of the message by the user as detected at step410, so that the certificate will be retrieved automatically, when it islikely that the user will be considering the message. Accordingly, theuser need not initiate retrieval of the requisite certificate manually.

Optionally, at step 450, the certificate retrieved at step 440 may beautomatically stored more permanently in a certificate store on themobile device, without user intervention (i.e. the user need not performmanual steps for the certificate to be stored). However, in a variantembodiment of the invention, some user input may be required by theapplication before the certificate is stored (e.g., the user may beprompted for a password to authorize the application to modify thecontents of the certificate store on the user's computing device).

In this embodiment, at step 460, the application also automaticallyverifies the digital signature using the certificate retrieved at step440, without user intervention (i.e. the user need not perform manualsteps to initiate verification of the digital signature). This step isperformed automatically as a convenience to the user. However, in avariant embodiment, the application may require that the user perform atleast one manual step to initiate verification of the digital signatureafter the certificate is retrieved at step 440.

At step 470, an indicator may be generated for display to the user thatindicates whether the digital signature was successfully verified atstep 460.

Referring now to FIG. 7B, a flowchart illustrating steps in a method ofverifying the revocation status of certificates associated with sendersof digitally signed messages received at a computing device in anembodiment of the invention is shown generally as 500. Further detailswith respect to various steps of method 500 have been discussed earlierin this specification.

In method 500, by way of example, the steps are described as beingperformed by an e-mail application that executes and resides on a mobiledevice. However, in variant embodiments, an application other than ane-mail application may perform the steps of the method. Furthermore, invariant embodiments, the application may be residing and executing on acomputing device other than a mobile device operated by a user, or on anintermediate server coupled to a mobile or other computing deviceoperated by a user, for example. The method may also be implementedthrough multiple applications executing and residing on the mobile orother computing device.

At step 510, the e-mail application detects the receipt by a user of adigitally signed message that comprises a digital signature of thesender of the message. In this embodiment, the message is considered asbeing received by the user when the user opens the message.

At step 520, the certificate associated with the sender that comprisesthe public key capable of verifying the digital signature is identifiedby the e-mail application and located on the mobile device. Informationrequired to identify the certificate may be provided within the receivedmessage. This information may include an identification of thecertificate's issuer and serial number, or the certificate's thumbprintfor example. This information can then be used to locate the requisitecertificate on the mobile device. The certificate may already have beenstored on the device before the message arrived at the device, or it mayhave been more recently retrieved from a certificate store (e.g. aremote LDAP server).

In a variant embodiment, the information required to identify thecertificate may be used to verify the revocation status of thecertificate at step 520 without requiring that the certificate belocated on the mobile device. In the event that the revocation status ofa certificate can be retrieved from a source of revocation status databy the application without requiring the certificate itself, an updatedrevocation status may be obtained without having to first retrieve theactual certificate (assuming that the certificate is not already storedon the mobile device). Certain manual and/or automated actions may thenbe performed in response. If the certificate has been revoked, theapplication might not automatically initiate retrieval of thecertificate, or the user might elect not to download the entire message,for example.

In this embodiment, step 520 is performed only after the user opens themessage. However, in a variant embodiment, this step may be performedwhen the message arrives at the device (e.g. when placed in an “inbox”folder for messages) but before the user opens the message. This is sothat when the user actually opens the message, the user need not waitfor this step to complete.

At step 530, the application can automatically initiate and perform averification of the revocation status of the certificate, if therevocation status of the certificate is not up-to-date. In thisembodiment, step 530 comprises determining the amount of time that haselapsed since the revocation status of the certificate was lastverified. This determination may be facilitated by comparing the currenttime with a time stamp associated with the certificate that identifiesthe last time the revocation status of the certificate was updated, forexample. Other means of tracking the elapsed time since the last updatemay be employed in variant embodiments.

Step 530 also comprises automatically retrieving the revocation status(e.g. from an OCSP server) if the determined amount of time exceeds apre-specified limit, which may have been established by the user ordefined by IT Policy.

Although other intervening steps (e.g. step 520) may be performedbetween step 510 and step 530, step 530 is effectively triggered by theopening of the message by the user as detected at step 510, so that therevocation status can be potentially updated (depending on the length oftime that has elapsed since the last update) automatically, every time auser opens (or re-opens) the message. The user need not initiate averification of the revocation status of the certificate manually.

At step 540, an indicator may be generated for display to the user thatindicates the result of the verification of the revocation status asperformed at step 530.

The methods described in FIGS. 7A and 7B need not both be implemented inan application executing on a given computing device. An applicationexecuting on a computing device may be programmed to perform the stepsof either one of these methods. Alternatively, the application may beprogrammed to perform a combination of the method steps as describedwith reference to FIG. 7C.

Referring to FIG. 7C, a flowchart illustrating steps in a method ofretrieving certificates associated with senders of digitally signedmessages received at a computing device in a variant embodiment of theinvention is shown generally as 600.

In this embodiment, the steps of the methods described in FIGS. 7A and7B are effectively combined. The task of retrieving certificates that isperformed by the application, and possibly a verification of acertificate's digital signature, is automatically triggered by thereceipt (e.g. opening) of a message (e.g. e-mail message) if thecertificate is not already stored on the computing device (e.g. mobiledevice), as was described with reference to FIG. 7A and steps 410through 470. Subsequently, once the certificate is retrieved,verification of the revocation status by the application isautomatically performed, as described with reference to FIG. 7B andsteps 530 through 540. In this manner, a number of tasks thattraditionally required manual initiation by users can be performedautomatically by the application without such user intervention.Accordingly, security with respect to digitally signed messages receivedby a user may be enhanced.

In a variant embodiment, steps related to the verification of therevocation status of a certificate may be performed prior to stepsrelated to the retrieval of the certificate.

In variant embodiments, the methods described with reference to FIGS. 7Ato 7C may further comprise the step of automatically initiating andperforming a verification of one or more other certificate properties,without user intervention (i.e. the user need not perform manual stepsto initiate the verifications). For example, such certificate propertiesmay include the trust status of a certificate, the expiration status ofthe certificate, and the strength of the public key of the certificate.An indicator may be generated for display to the user that indicates aresult of the verification of the one or more certificate properties.

The steps of the methods described herein may be provided as executablesoftware instructions stored on computer-readable media, which mayinclude transmission-type media.

The invention has been described with regard to a number of embodiments.However, it will be understood by persons skilled in the art that othervariants and modifications may be made without departing from the scopeof the invention as defined in the claims appended hereto.

1. A method of retrieving certificates associated with senders ofdigitally signed electronic mail messages received at a user's mobilecomputing device, wherein the method is performed by an applicationexecuting on the user's mobile computing device, the method comprising:after an electronic mail message comprising a digital signature of asender arrives at the user's mobile computing device, before theelectronic mail message is user-selected for opening, and beforeverifying the digital signature: identifying a certificate associatedwith the sender that comprises a public key capable of verifying thedigital signature, wherein the certificate is not included in theelectronic mail message; and determining whether the certificate isstored in a certificate store on the user's mobile computing device;retrieving the certificate from a certificate store remotely locatedfrom the user's mobile computing device if the certificate is determinedto be not stored on the user's mobile computing device; verifying, bythe user's mobile computing device, the digital signature of theelectronic mail message using the certificate; and generating anindicator for display at the user's mobile computing device thatindicates whether the digital signature is successfully verified.
 2. Themethod of claim 1, wherein the application comprises an electronic mailapplication.
 3. The method of claim 1, wherein the retrieving isperformed by the user's mobile computing device after the determining,without user intervention.
 4. The method of claim 1, further comprisingstoring the certificate retrieved from the certificate store remotelylocated from the user's mobile computing device, in a certificate storeon the user's mobile computing device.
 5. The method of claim 3, whereinsaid storing the certificate is performed by the application after thecertificate is retrieved from the certificate store remotely locatedfrom the user's mobile computing device, without user intervention. 6.The method of claim 1, wherein the verifying and generating areperformed after the retrieving without user intervention.
 7. The methodof claim 1, further comprising verifying at least one certificateproperty of the certificate, and generating an indicator that indicate aresult of the verifying.
 8. The method of claim 7, wherein the at leastone certificate property comprises the trust status of the certificate.9. The method of claim 7, wherein the at least one certificate propertycomprises the strength of the public key of the certificate.
 10. Themethod of claim 7, wherein the at least one certificate propertycomprises the expiration status of the certificate.
 11. A mobile devicecomprising: a processor, and a memory; wherein the processor isconfigured to: after an electronic mail message comprising a digitalsignature of a sender arrives at the mobile, before the electronic mailmessage is user-selected for opening, and before verifying the digitalsignature: identify a certificate associated with the sender thatcomprises a public key capable of verifying the digital signature,wherein the certificate is not included in the electronic mail message;and determine whether the certificate is stored in a certificate storeon the mobile device; retrieve the certificate from a certificate storeremotely located from the mobile device if the certificate is determinedto be not stored on the mobile device; verify the digital signature ofthe electronic mail message using the certificate; and generate anindicator for display at the mobile device that indicates whether thedigital signature is successfully verified.
 12. The device of claim 11,wherein the processor is configured to retrieve the certificate afterdetermining that the certificate is not stored on the mobile device,without user intervention.
 13. The device of claim 11, wherein theprocessor is configured to store the certificate retrieved from thecertificate store remotely located from the mobile device, in acertificate store on the mobile device.
 14. The device of claim 13,wherein the processor is configured to store the certificate after thecertificate is retrieved from the certificate store remotely locatedfrom the mobile device, without user intervention.
 15. The device ofclaim 11, wherein the processor is configured to verify the digitalsignature and generate the indicator for display after retrieving thecertificate, without user intervention.
 16. The device of claim 11,wherein the processor is configured to verify at least one certificateproperty of the certificate, and generate an indicator that indicate aresult of verifying the at least one certificate property.
 17. Thedevice of claim 16, wherein the at least one certificate propertycomprises the trust status of the certificate.
 18. The device of claim16, wherein the at least one certificate property comprises the strengthof the public key of the certificate.
 19. The device of claim 16,wherein the at least one certificate property comprises the expirationstatus of the certificate.
 20. A computer-readable storage device,comprising a plurality of instructions executable by a processor of amobile device, wherein the instructions configured the processor to:after an electronic mail message comprising a digital signature of asender arrives at the mobile, before the electronic mail message isuser-selected for opening, and before verifying the digital signature:identify a certificate associated with the sender that comprises apublic key capable of verifying the digital signature, wherein thecertificate is not included in the electronic mail message; anddetermine whether the certificate is stored in a certificate store onthe mobile device; retrieve the certificate from a certificate storeremotely located from the mobile device if the certificate is determinedto be not stored on the mobile device; verify the digital signature ofthe electronic mail message using the certificate; and generate anindicator for display at the mobile device that indicates whether thedigital signature is successfully verified.